1. Field of the Invention
The present invention relates to a throttle valve control system for controlling the opening of the throttle valve of an internal combustion engine.
2. Description of Background Information
Various measures have been proposed for improving fuel economy of an internal combustion engine. The lean burn system characterized by the supply of a lean air-fuel mixture, the generation of swirling action in the combustion chamber for enhancing sufficient mixture flow in the combustion chamber, or the improvement of the shape of the combustion chamber, are examples of such measures. However, in those systems, there has been a problem that the structure of the system generally becomes complicated, so that an increase of the cost is not avoided. One method for solving the above problem is to provide a system for controlling the opening of the throttle valve of the engine. For example, Japanese Patent Application Laid Open No. P60-192843 discloses a control of the throttle valve opening in which the throttle valve opening is determined simply ,in accordance with the operation position of the accelerator pedal and the engine rotational speed. Therefore, the improvement of the fuel economy was not gained by such a throttle valve opening control.
Furthermore, with a vehicle equipped with a system for controlling the engine rotational speed in accordance with the operating position of the accelerator pedal such as the CVT (continuously variable transmission), the engine rotational speed does not rise rapidly due to a delay of response of the system to the control operation even if the throttle valve is depressed rapidly for accelerating the vehicle. In the case of an arrangement in which the throttle valve opening is always controlled to a target throttle opening determined correspondingly to the engine speed, the throttle valve opening can not be increased rapidly, making it impossible to secure good acceleration of the vehicle. Therefore, it is desirable to devise a suitable measure for preventing such a problem.
On the other hand, in a region in which the throttle valve opening is small, the amount of engine intake air varies widely with respect to a smaller change in the throttle valve position than in a state in which the throttle valve opening is medium or high. Therefore, if the throttle valve is controlled indirectly, for example by using a motor all the time, a problem may arise that the driveability of the engine becomes unsatisfactory with respect to delicate operations of the accelerator pedal by the driver. Moreover, hunting of the throttle opening with respect to a control throttle valve opening due to a delay of response to the control operation, makes it difficult to maintain a good driveability of the engine. Therefore, it is desirable to provide a suitable measure for preventing this problem.